Integrated photonics vertical coupler

ABSTRACT

Systems and methods for an integrated photonics vertical coupler are provided herein. In certain embodiments, a device includes a first waveguide having a first photon and a second photon propagating therein, wherein the first photon and the second photon are propagating in orthogonal modes. Further, the device includes a second waveguide having a second coupling portion in close proximity with a first coupling portion of the first waveguide, wherein a physical relationship between the first waveguide and the second waveguide along the length of the second coupling portion causes an adiabatic transfer of the first photon and the second photon into distinct orthogonal modes of the second waveguide at different locations in the second coupling portion.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application Ser.No. 62/924,058, filed Oct. 21, 2019, and titled “INTEGRATED PHOTONICSSOURCE AND DETECTOR OF ENTANGLED PHOTONS,” which is hereby incorporatedherein by reference.

BACKGROUND

Networks of synchronized atomic clocks are frequently used to distributeaccurate time across distances. For example, the global navigationsatellite systems (GNSS) such as the global position system (GPS),GLONASS, BeiDou, and Galileo are comprised of satellites withsynchronized atomic clocks and provide the distribution of internationaltime. Often, satellites are equipped with hardware to facilitate thesynchronization of clocks on separate satellites. Synchronizationhardware of reduced size and weight, and capable of high precisiontiming alignment, permits synchronizing of atomic clocks of smallersatellites.

SUMMARY

Systems and methods for an integrated photonics vertical coupler areprovided herein. In certain embodiments, a device includes a firstwaveguide having a first photon and a second photon propagating therein,wherein the first photon and the second photon are propagating inorthogonal modes. Further, the device includes a second waveguide havinga second coupling portion in close proximity with a first couplingportion of the first waveguide, wherein a physical relationship betweenthe first waveguide and the second waveguide along the length of thesecond coupling portion causes an adiabatic transfer of the first photonand the second photon into distinct orthogonal modes of the secondwaveguide at different locations in the second coupling portion.

DRAWINGS

Understanding that the drawings depict only some embodiments and are nottherefore to be considered limiting in scope, the exemplary embodimentswill be described with additional specificity and detail using theaccompanying drawings, in which:

FIG. 1 is a block diagram illustrating an exemplary interferometeraccording to an aspect of the present disclosure;

FIG. 2 is a diagram illustrating different paths in a chip-scale deviceaccording to an aspect of the present disclosure;

FIGS. 3A-3C are diagrams showing the paths through the chip-scale devicefor the different modes according to an aspect of the presentdisclosure;

FIG. 4 is a diagram illustrating the various components in a chip-scaledevice according to an aspect of the present disclosure;

FIG. 5 is a diagram illustrating a vertical coupler according to anaspect of the present disclosure;

FIG. 6 is a diagram illustrating a mode splitter according to an aspectof the present disclosure;

FIG. 7 is a diagram illustrating a mode converter according to an aspectof the present disclosure;

FIG. 8 is a diagram illustrating a bandpass filter according to anaspect of the present disclosure;

FIG. 9 is a flowchart diagram illustrating an exemplary method for usinga chip-scale device to perform interferometry according to an aspect ofthe present disclosure; and

FIG. 10 is a flowchart diagram illustrating an exemplary method forvertically coupling photons from a first waveguide to a secondwaveguide.

In accordance with common practice, the various described features arenot drawn to scale but are drawn to emphasize specific features relevantto the example embodiments.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings that form a part hereof, and in which is shown byway of illustration specific illustrative embodiments. However, it is tobe understood that other embodiments may be utilized and that logical,mechanical, and electrical changes may be made.

Systems and methods for an integrated photonics source and detector ofentangled photons are provided herein. In certain embodiments, hardwareis described herein that enables methods for precise and securesynchronization of optical atomic clocks using the quantum interferenceof time-entangled photons. For example, the optical atomic clocks onorbiting satellites may be precisely and securely synchronized. Deployedacross a swarm of LEO/MEO satellites, embodiments described herein mayenable improved modalities of signal intelligence based on the coherentcombination of distributed radio or optical apertures, includingreal-time computational interferometry for increased sensitivity to weaksignals, and active beam forming radar/imaging for increased covertnessby reducing both signal spillover and time-on-target.

Additionally, clock synchronization schemes, described herein, may use achip-scale, ultra-high-flux source and interferometer for time-energyentangled bi-photons, with a reduced size, weight, and power, high pairproduction rate, and high flux-to-background ratio for entangled photonpairs. Also, for increased size, weight, and power reduction andimproved deployability in small satellite platforms, devices describedherein may be integrated onto a chip. In particular, both a photonsource and interferometric detector may be integrated onto a chip.

In certain embodiments, entangled photons may be generated through aspontaneous parametric degenerate down-conversion of pump photons, alsoknown as degenerate difference frequency generation. Typically, theabove method for photon generation may yield entangled photons that haveorthogonal polarizations to one another. Typically, free-space opticsare used to separate the entangled photons and convert them into thesame polarization state for use within a clock synchronization scheme.Embodiments described herein provide a chip-scale photonic integratedcircuit having on-chip guided wave photonics for separating theentangled photons and converting the separated photons into the samepolarization state.

In some embodiments, a chip-scale photonic integrated circuit mayproduce and interfere time-entangled photons. The chip-scale photonicintegrated circuit may realize the optical functions for producing andinterfering the photons on a hybrid optical waveguide platform whichcombines the nonlinear properties of periodically poled potassiumtitanyl phosphate (ppKTP) waveguides or waveguides made from similarmaterial to ppKTP with the low transmission loss, high confinement, andfiltering capabilities of silicon nitride waveguides or other waveguidesmade from similar material to silicon nitride. The chip-scale approachusing the combination of waveguides made from different materials enableimprovements over previous types of sources based in fiber andfree-space optics.

In some embodiments, materials that have both nonlinear properties andlow transmission loss, high confinement, and filtering capabilitiescould be used to implement similar optical functions for producing andinterfering the photons in an optical waveguide platform based on asingle material system, such as lithium niobate.

In certain embodiments, the optical functionality for producing andreceiving entangled photons is implemented on a single, integratedplatform, yielding reduced optical losses, enhanced mode overlap,efficient filtering of photons, increased interferometer contrast, andimproved mechanical robustness, all while reducing size, weight, andpower when compared to fiber or free space based systems. Additionally,embodiments described herein permit higher precision timesynchronization when used in a system while enabling usage on smallersatellite platforms, such as microsats.

FIG. 1 is a diagram illustrating a system 100 for a Hong-Ou-Mandel (HOM)interferometer. As used herein, the chip scale integrated circuit may beused within a HOM interferometer. As used herein, a HOM interferometeris a device that may produce a pump photon 101. The system 100 may splitthe pump photon into two daughter photons 103 (referred to separatelyherein as photons 103-A and 103-B). For example, the pump photon 101 maybe produced by a laser source that produces a laser having a wavelengthof 405 nm or other desired wavelength.

In certain embodiments, the pump photon 101 is split into daughterphotons 103 that are guided through optical structures forrecombination. For example, the pump photon 101 is split by opticalstructure 105 into daughter photons 103-a and 103-b. The daughterphotons 103 may each have a wavelength that is twice the wavelength ofthe pump photon 101 (i.e., where the pump photon 101 could have awavelength of 405 nm, the daughter photons 103 may each have awavelength of 810 nm). Additionally, the system 100 may include guidingoptics 107 that guide the daughter photons 103 to beamsplitter 110, uponwhich the daughter photons 103 are combined, such that quantumsuperpositions 103-c and 103-d of the daughter photons impinge ondetectors 109 for reception. For example, a detector 109-a may receiveand detect the daughter photon 103-a and the detector 109-b may receiveand detect the daughter photon 103-b; or detector 109-a may receive anddetect the daughter photon 103-b and the detector 109-b may receive anddetect the daughter photon 103-a or detector 109-a may receive anddetect both daughter photons 103-a and 103-b; or detector 109-b mayreceive and detect both daughter photons 103-a and 103-b, in the mannerof a HOM interferometer.

In some embodiments, when the detectors 109 receive the associateddaughter photons 103, the detectors 109 may provide the signals to anelectronic correlator device 111, where the electronic correlator device111 combines the electrical signals of the two detectors 109 for theperformance of HOM interferometry. The electronic correlator device 111quantitively determines the degree of temporal correlation of thesignals produced by the detectors 109. For example, the electroniccorrelator 111 may show that the coincidence rate of the signalsprovided by the photodetectors 109 may drop towards zero when thedaughter photons 103 overlap substantially perfectly in time. This droptowards a zero rate of coincident detections is known as the HOM dipillustrated in the trace graph 113. The dip occurs when the two daughterphotons 103 are substantially identical in all properties. When thephotons 103 become distinguishable, including and especially in regardsto the equality of their times-of-flight between the source region 105and the beam splitter 110, the HOM dip disappears. In this way thesystem 100 is sensitive to the quality of the times-of-flight of thedaughter photons 103 between the source region 105 and the beam splitter110 being substantially perfectly equal.

FIG. 2 illustrates different optical paths 201 and 203 on a chip-scaledevice 200 that is both capable of generating a photon, splitting thephoton into daughter photons, providing the daughter photons as outputs(such as into free space or optical fibers), receiving the daughterphotons which may have been reflected from remote mirrors or opticalsystems, and providing the received photons to an interferometer forperforming HOM interferometry. As shown, FIG. 2 illustrates a sourcepath 201 and an interferometer path 203. In the source path 201, anincoming pump photon is split into daughter photons which may beseparated and directed to different remote platforms. In theinterferometer path 203, the daughter photons reflected by the remoteplatforms are received and interfered in the manner of HOMinterferometry.

In certain embodiments, the chip-scale device 200 utilizes the nonlinearoptical effect of degenerate spontaneous parametric down conversion(dSPDC), in which a pump photon 205 splits into two “twin” daughterphotons 209 and 211 that are “born” at nearly the same instant (e.g.,within <100 femtoseconds of one another). This simultaneity, enforced byquantum mechanics, may be exploited for synchronizing separated atomicclocks. To synchronize the separated atomic clocks, (i.e., when thedifferent atomic clocks are located on different satellites) thesynchronization is achieved by projecting daughter photons 209 and 211from the chip-scale device 200, reflecting some of the photons 209 and211 from each of the satellites, and providing them for recombining in aHong-Ou-Mandel (HOM) interferometer 215, in which a purely quantummechanical interference “dip” in the coincidence rate is observed onlywhen the paths are substantially exactly equal as described above withrespect to FIG. 1. The arrival times of some of the entangled photonsfrom each satellite may be compared over a classical channel, enablingcontrollers to synchronize the clocks with great precision (i.e.,potentially with femtosecond precision).

In some embodiments, the chip scale device 200 is a chip-scale photonicintegrated circuit that produces and interferes time-entangled photons.The chip-scale device 200 may include optical functions and componentson a hybrid optical waveguide platform which combines the nonlinearproperties of ppKTP waveguides (or other waveguides made from materialshaving similar properties) with the high confinement and filteringcapabilities of silicon nitride waveguides. This combination permitsminiaturization, efficiency, robustness, while increasing the useableflux of twin-photons 209 and 211.

In some embodiments, the chip-scale device 200 may include opticalfunctions and components on a single optical waveguide material platformthat has both nonlinear properties and low transmission loss, highconfinement, and filtering capabilities, such as lithium niobate.

In certain embodiments, the chip scale device may generate a pump photon205, and from the pump photon 205 in the source path 201, and maygenerate, by dSPDC, daughter photons 206 a and 206 b in the photonproducing waveguide. Each of the twin photons 206 a and 206 b may occupya different waveguide mode, either Transverse Electric (TE), orTransverse Magnetic (TM). A Vertical Coupler (VC) region mayadiabatically draw the daughter photons 206 a and 206 b out of thephoton producing waveguide and into a photon conditioning waveguidepatterned on top of the photon producing waveguide. Additionally, the TMand TE photons may be separated by two diffractive waveguide modesplitters (MS). The TE photon may then pass through a bandpass filter(BPF) to reject background photons, through a second MS, then may leavethe chip 200 as emitted photon 211. Meanwhile, the original TM photonmay be converted into a TE mode by a diffractive mode converter (MC),which may also reverse the direction of propagation of the photon. This(now TE polarized) photon may pass through its own bandpass filter andleave the chip 200 as emitted photon 209. The various functionsperformed on the chip may be performed by a photon conditioningwaveguide (in some embodiments, made from silicon nitride or othersimilar material), where waveguide structures are patterned in a filmdeposited on top of the substrate containing the photon producingwaveguide.

In additional embodiments, the interferometer path 203. the twin-photons209 and 211 may be reflected or sent back from remote satellites orother remote systems and are recoupled into the photonics componentwaveguides on the chip-scale device 200 to complete an HOMinterferometer 215. (In some implementations, the photons may also havetheir polarizations rotated by 90 degrees by conventional waveplates).Although the twin photons 209 and 211 may re-enter the same waveguidesfrom which they were earlier emitted, because of their now rotatedpolarizations, they may couple into the orthogonal waveguide mode (i.e.,TM). Each photon then may interact with a diffractive mode splitter (MS)that may reverse the direction of propagation in the waveguide, sendingthe photons 209 and 211 to the 50/50 waveguide coupler. The output portsof the interferometer may be directed onto photon detectors 212, such assingle-photon avalanche photodetectors (SP-APDs), where the photons 209and 211 may be detected. The detected signal outputs of the photondetectors 212 may be directed to an electronic correlator 215, which maydetermine the degree of coincidence of the arrive times of the signals,thus completing an HOM interferometer 216.

FIGS. 3A-3C illustrate the propagation of the two photons produced bythe photon producing waveguide, into a photon vertical couplingwaveguide, and through the photon conditioning waveguide network. Asdiscussed above, the photon producing waveguide produces two photonshaving orthogonal waveguide modes: one mode propagating in the TM modeand the other propagating in the TE mode. Depending on the mode of thephoton, the photons propagate along different paths through thewaveguide network, such that the waveguide network provides two photonspropagating in the TE mode off of the chip and receives two photons backonto the chip, propagating in the TM mode. FIG. 3A illustrates the pathof the photon originally in the TE mode of the photon vertical couplingwaveguide 302. FIG. 3B illustrates the path of the photon originally inthe TM mode of the photon vertical coupling waveguide 302. FIG. 3Cillustrates the path of the photons through the photon conditioningwaveguide network 304 that are received from external devices.

In certain embodiments illustrated in FIG. 3A, the photon in the TE modeof the photon vertical coupling waveguide 302, passing into the photonconditioning waveguide network 304, passes through a mode splitter 303without diffraction. Then the photon passes through a bandpass filter305, which filters out fluorescence, as well as stray pump light coupledfrom the photon producing waveguide 301. The photon then passes throughthe mode splitter 307 without diffraction and is emitted through theoutput port 321.

In certain embodiments illustrated in FIG. 3B, the photon in the TM modeof the photon vertical coupling waveguide 302, passing into the photonconditioning waveguide network 304, is diffracted by the mode splitter303. The photon is further diffracted by the mode splitter 309,whereupon the photon enters the mode converter 311. The mode converter311 again diffracts the photon but converts the photon from the TM modeinto the TE mode. As the photon is now in the TE mode, the photon is notdiffracted by the mode splitter 309. The photon then passes through thebandpass filter 313, which filters out fluorescence, as well as straypump light coupled from the photon producing waveguide. The photon thenpasses through the mode splitter 315 without diffraction and is emittedthrough the output port 319.

In additional embodiments illustrated in FIG. 3C, the two daughterphotons emitted from the photon conditioning waveguide network 304 maybe sent back from another optical device such that they are recoupledinto the photon conditioning network 304 in TM modes at the waveguides319 and 321. The two received photons in the TM modes may propagate intothe waveguides to the mode splitters 315 and 307 respectively. Both modesplitters 315 and 307 diffract the received photons. The photons thenare interfered with one another via a 50/50 coupler 317 before beingoutput on ports 323 and 325 for subsequent detection by photondetectors. In embodiments discussed above, the TM mode from the photonvertical coupling waveguide 302 is converted by the photon conditioningwaveguide network 304 into a TE mode for transmission out of thechip-scale device, while the light received back into the device forsubsequent interferometric detection is in the TM mode. However, inanother embodiment, the TE mode from the photon vertical couplingwaveguide is converted by the photon conditioning waveguide network 304into a TM mode for transmission out of the chip-scale device, while thelight received back into the device for subsequent interferometricdetection is in the TE mode.

FIG. 4 illustrates the different photonics components within thechip-scale device 400. For example, the chip scale device 400 includes aphoton producing waveguide 401, a photon vertical coupling waveguide427; and a photon conditioning waveguide network (similar to the photonconditioning waveguide network 304 in FIGS. 3A-3C) comprising modesplitters, 403, 409, 407, and 415; mode converter 411; bandpass filters413 and 405; input/output waveguides 419, 421, 423, and 425; and 50/50coupler 417. Possible embodiments for the vertical coupler 427; modesplitters 403, 409, 407, and 415; mode converter 411, and bandpassfilters are described in greater detail below.

FIG. 5 is a side view diagram illustrating the operation of a verticalcoupler. To efficiently couple photons out of photon producing waveguide501 into the photon vertical coupling waveguide 503, a stacked waveguideis formed. Further, a relatively thin photon vertical coupling waveguide503 in relation to the width of the photon producing waveguide 501 haslittle perturbation on the shape of the weakly confined modes in thephoton producing waveguide 501. As discussed herein, the photon verticalcoupling waveguide 503 is gradually widened throughout the overlappingportions of the stacked waveguide. For example, the photon verticalcoupling waveguide 503 may widen from 100 nm to 200 nm over a distanceof ˜500 microns. The gradual widening of the photon vertical couplingwaveguide 503 adiabatically draws the photons from the photon producingwaveguide 501 into a much more tightly confined waveguide mode.Additionally, the transfer preserves the polarization modes of thepropagating photons (i.e., TE→TE, and TM→TM), with essentially zero modecross-coupling. As such photons propagating in different modes may becoupled out of the photon producing waveguide 501 at differentlocations. Accordingly, as different modes may be coupled from out ofthe photon producing waveguide 501 at different locations, the verticalcoupler may be implemented in other applications such as in a modesplitter and the like.

In further embodiments, the material used to produce the photonproducing waveguide and the material used to produce the photon verticalcoupling waveguide may have a large difference between their respectiveindexes of refraction. For example, where KTP is used for the photonproducing waveguide, the photon vertical coupling waveguide may be madeusing silicon enriched nitride films.

FIG. 6 is a diagram illustrating certain aspects of a mode splitter asfound in the chip-scale device 200. In particular, FIG. 6 shows anisometric view 600 of a mode splitter, a detailed isometric view 610 ofa portion of the mode splitter, and a frequency response graph 620 ofthe coupling of the different modes within the mode splitter.

In certain embodiments, as shown in the isometric view 600, the modesplitter may include a single input port 603. Through the input port themode splitter may receive two photons as an input 601 that arepropagating in different orthogonal modes within the waveguide. Forexample, one photon may be propagating in the TE mode and another photonmay be propagating in the TM mode. The mode splitter may pass one of thereceived photons at the input port 603 through to the output port 607 asan output photon 609. For example, the mode splitter may pass the TEmode photon received at the input port 603 directly through to theoutput port 607. Additionally, the mode splitter may diffract one of thepropagating photons so that one of the propagating photons is coupledinto a contra-directional waveguide and passed through to the outputport 613 as output 611. For example, the TM mode may be diffracted by acoupling portion 605 of the mode splitter and passed to the output port613.

In some embodiments, as shown in the detailed isometric view 610 of thecoupling portion 605 of the mode splitter, to split the two orthogonallypolarized photons into different paths, the mode splitter may include achirped-grating-assisted contra-directional mode coupler. As shown, view610 depicts the waveguide structure and graph 620 shows the results of acalculation of its spectral response. As shown, the coupling portionconsists of two closely spaced waveguides 621 and 625. The waveguide 621may further be patterned with a modulated sidewall 623, thus, creatingan in-waveguide diffraction grating which has a large overlap integralfor the TM-to-TM transition from one waveguide to the other. The effectof the modulation is to couple the TM mode from the forward direction inthe waveguide 625 to the backward direction in the waveguide 621;whereas the TE mode passes through the mode splitter in the forwarddirection, remaining in waveguide 625. Additionally, the frequency ofthe modulated sidewall 623 may change along the length of the modesplitter, to allow for a desired frequency response for the modesplitter.

FIG. 7 is a diagram illustrating certain aspects of a mode converter asfound in the chip-scale device 200. In particular, FIG. 7 shows anisometric view 700 of a mode splitter, a detailed isometric view 710 ofa converting portion of the mode splitter, and a frequency responsegraph 720 of the conversion of the modes within the mode converter.

In certain embodiments, as shown in the isometric view 700, the modeconverter may include a single port 703. Through the port 703 the modeconverter may receive a photon as an input 701 that is propagating in aparticular mode within the waveguide. For example, the photon receivedthrough the port 703 may be propagating in the TM mode. The modeconverter may convert the mode from one mode into an orthogonal modewithin a converting portion 705, where the mode converter converts thephoton into an orthogonally propagating mode to be output through theport 703 as an output. For example, when the photon received on the port703 is in the TM mode, the photon output through the port 703 may be inthe TE mode.

In some embodiments, as shown in the detailed isometric view 710 of theconverting portion 705 of the mode converter, to make all the waveguidepaths as similar as possible for the two photons, the chip-scale devicemay flip the in-waveguide polarization of the TM photon using a singlewaveguide grating structure designed with asymmetrically modulatedsidewalls 709 and 711. For example, the modulation of the sidewalls maybe out of phase with each other such that the transverse cross-sectionof the waveguide along the length of the modulation is constant. Thisasymmetric modulation creates a cross coupling between the TM mode inthe forward direction and the TE mode in the backward direction. Asshown in the graph 720, Mode conversion only occurs within the stopbandof the grating. To control the stopbands of the grating, the length ofthe converting portion 705 may be changed along with the modulationfrequency of the modulated sidewalls 709 and 711. For example, thefrequency of the modulated sidewalls may either decrease or increasealong the length of the converting portion of the mode converter.

FIG. 8 is a diagram illustrating certain aspects of a bandpass filter asfound in the chip-scale device 200. In particular, FIG. 8 shows anisometric view 800 of a bandpass filter, a detailed isometric view 810of a filtering portion of the bandpass filter, and a frequency responsegraph 820 of the filtering of photons by the bandpass filter.

In certain embodiments, as shown in the isometric view 800, the bandpassfilter may include a single port 803. Through the port 803, the bandpassfilter may receive a photon as an input 801 that is propagating in aparticular mode within the waveguide. For example, the photon receivedthrough the input port 803 may be propagating in the TE mode. Thebandpass filter may filter photons having unwanted wavelengths in afiltering portion 805 and provide the filtered photons as output 809through the output port 807.

In some embodiments, as shown in the detailed isometric view 810 of thefiltering portion 805 of the bandpass filter, to reject any backgroundfluorescence photons that may be propagating in the waveguides, as wellas to reject any residual pump photons, a waveguide bandpass filter isimplemented. As shown, the filter is made from two high reflectivitywaveguide gratings 811 and 813 that manifest by a chirp in themodulation period along the length of the waveguide, in other words, themodulation of the waveguide gratings symmetrically, longitudinallyvaries along the length of the sidewalls of the filters. Light justoutside of the passband is diffracted back down the waveguide, whilelight at the pump wavelength is scattered out of the waveguide entirely.In some embodiments the spectral location of the waveguide gratings 811and 813 may change along the length of the filtering portion 805 of thepassband.

FIG. 9 is a method 900 of using a chip-scale device to produce andinterfere pairs of correlated photons, as described above. The method900 proceeds at 901, where a pair of photons are generated in a photonproducing waveguide. Additionally, the method 900 proceeds at 903, wherethe pair of photons is coupled into a photon vertical couplingwaveguide. Further, the method 900 proceeds at 905, where one of thephotons in the pair of photons is converted in a photon conditioningwaveguide network so that photons are propagating in identical modes intwo different waveguides. In certain embodiments, the method 900proceeds at 907, where the photons are provided to one or more externaldevices. Further, the method 900 proceeds at 909, where the photons arereceived from the one or more external devices. Additionally, the method900 proceeds at 911, where interferometry is performed on the receivedphotons.

FIG. 10 is a method 1000 for vertically coupling two photons from afirst waveguide into a second waveguide. The method 1000 proceeds at1001, where a first photon and a second photon are generated in a firstwaveguide in a first waveguide layer. Further, the first photon and thesecond photon may be in different modes that are orthogonal to oneanother. For example, the first photon may be propagating in the TE modeand the second photon may be propagating in the TM mode. Additionally,the method 1000 proceeds at 1003, where the first photon is coupled fromthe first waveguide into a second waveguide at a first location within acoupling portion of the second waveguide. Moreover, the method 1000proceeds at 1005, where the second photon is coupled from the firstwaveguide into the second waveguide at a second location distinct fromthe first location within the coupling portion. For example, the firstphoton and the second photon are coupled into one of the first locationand the second location based on the mode of propagation within thefirst waveguide.

EXAMPLE EMBODIMENTS

Example 1 includes a device comprising: a first waveguide having a firstphoton and a second photon propagating therein, wherein the first photonand the second photon are propagating in orthogonal modes; and a secondwaveguide having a second coupling portion in close proximity with afirst coupling portion of the first waveguide, wherein a physicalrelationship between the first waveguide and the second waveguide alongthe length of the second coupling portion causes an adiabatic transferof the first photon and the second photon into distinct orthogonal modesof the second waveguide at different locations in the second couplingportion.

Example 2 includes the device of Example 1, wherein the adiabatictransfer of the first photon and the second photon into the secondwaveguide preserves the orthogonal modes of the first photon and thesecond photon when propagating in the first waveguide.

Example 3 includes the device of any of Examples 1-2, wherein the firstphoton is in a TE mode and the second photon is in a TM mode.

Example 4 includes the device of any of Examples 1-3, wherein the firstphoton is coupled into the second coupling portion before the secondphoton.

Example 5 includes the device of any of Examples 1-4, wherein the firstwaveguide is formed in a first waveguide layer and the second waveguideis formed in a second waveguide layer, wherein the first waveguide layerand the second waveguide layer are made from materials having differentindexes of refraction.

Example 6 includes the device of Example 5, wherein the first waveguidelayer is made of periodically poled potassium titanyl phosphate.

Example 7 includes the device of any of Examples 5-6, wherein the secondwaveguide layer is made of silicon nitride.

Example 8 includes the device of any of Examples 1-7, wherein the firstphoton and the second photon are generated within the first waveguide.

Example 9 includes the device of any of Examples 1-8, wherein thephysical relationship comprises changing a width of the second waveguidealong the length of the second coupling portion.

Example 10 includes the device of Example 9, wherein the width graduallychanges by widening along the direction of propagation of the firstphoton and the second photon within the second waveguide.

Example 11 includes a device comprising: a first waveguide layer havinga first waveguide therein, the first waveguide having a first photon anda second photon propagating therein, wherein the first photon and thesecond photon are propagating in orthogonal modes; and a secondwaveguide layer having a second waveguide therein, the second waveguidehaving a second coupling portion in close proximity with a firstcoupling portion of the first waveguide, wherein the first photon andthe second photon are adiabatically transferred into distinct orthogonalmodes of the second waveguide, wherein the first waveguide layer and thesecond waveguide layer are made of materials having different indexes ofrefraction.

Example 12 includes the device of Example 11, wherein width of thesecond waveguide changes along the length of the second couplingportion.

Example 13 includes the device of any of Examples 11-12, wherein thewidth gradually changes by widening along the direction of propagationof the first photon and the second photon within the second waveguide.

Example 14 includes the device of any of Examples 11-13, wherein thefirst photon and the second photon are coupled into the second waveguideat different locations in the second coupling portion.

Example 15 includes the device of any of Examples 10-14, wherein thefirst photon is coupled into the second coupling portion before thesecond photon.

Example 16 includes the device of any of Examples 10-15, wherein thefirst waveguide layer is made of periodically poled potassium titanylphosphate.

Example 17 includes the device of any of Examples 10-16, wherein thesecond waveguide layer is made of silicon nitride.

Example 18 includes a method comprising: generating a first photon and asecond photon in a first waveguide formed in a first waveguide layer,where the first photon is in a first mode and the second photon is in asecond mode orthogonal to the first mode; coupling the first photon fromthe first waveguide into a second waveguide at a first location within acoupling portion of the second waveguide, wherein the coupling portionis a section of the second waveguide proximate to the first waveguide;and coupling the second photon from the first waveguide into the secondwaveguide at a second location distinct from the first location withinthe coupling portion, wherein the first photon and the second photon arecoupled into one of the first location and the second location based onwhether propagation within the first waveguide is in the first mode orthe second mode.

Example 19 includes the method of Example 18, wherein a width of thesecond waveguide changes along the length of the coupling portion bywidening along the direction of propagation of the first photon and thesecond photon within the second waveguide.

Example 20 includes the method of any of Examples 18-19, wherein thefirst waveguide is formed in a first waveguide layer and the secondwaveguide is formed in a second waveguide layer, wherein the firstwaveguide layer and the second waveguide layer are made from materialshaving different indexes of refraction.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement, which is calculated to achieve the same purpose,may be substituted for the specific embodiments shown. Therefore, it ismanifestly intended that this invention be limited only by the claimsand the equivalents thereof.

What is claimed is:
 1. A device comprising: a first waveguide having afirst photon and a second photon propagating therein, wherein the firstphoton and the second photon are propagating in orthogonal modes,wherein the first photon and the second photon are generated within thefirst waveguide; and a second waveguide having a second coupling portionin close proximity with a first coupling portion of the first waveguide,wherein a physical relationship between the first waveguide and thesecond waveguide along thea length of the second coupling portion causesan adiabatic transfer of the first photon and the second photon intodistinct orthogonal modes of the second waveguide at different locationsin the second coupling portion.
 2. The device of claim 1, wherein theadiabatic transfer of the first photon and the second photon into thesecond waveguide preserves the orthogonal modes of the first photon andthe second photon when propagating in the first waveguide.
 3. The deviceof claim 1, wherein the first photon is in a TE mode and the secondphoton is in a TM mode.
 4. The device of claim 1, wherein the firstphoton is coupled into the second coupling portion before the secondphoton.
 5. The device of claim 1, wherein the first waveguide is formedin a first waveguide layer and the second waveguide is formed in asecond waveguide layer, wherein the first waveguide layer and the secondwaveguide layer are made from materials having different indexes ofrefraction.
 6. The device of claim 5, wherein the first waveguide layeris made of periodically poled potassium titanyl phosphate.
 7. The deviceof claim 5, wherein the second waveguide layer is made of siliconnitride.
 8. The device of claim 1, wherein the physical relationshipcomprises changing a width of the second waveguide along the length ofthe second coupling portion.
 9. The device of claim 8, wherein the widthgradually changes by widening along thea direction of propagation of thefirst photon and the second photon within the second waveguide.
 10. Adevice comprising: a first waveguide layer having a first waveguidetherein, the first waveguide having a first photon and a second photonpropagating therein, wherein the first photon and the second photon arepropagating in orthogonal modes, wherein the first photon and the secondphoton are generated within the first waveguide; and a second waveguidelayer having a second waveguide therein, the second waveguide having asecond coupling portion in close proximity with a first coupling portionof the first waveguide, wherein the first photon and the second photonare adiabatically transferred into distinct orthogonal modes of thesecond waveguide, wherein the first waveguide layer and the secondwaveguide layer are made of materials having different indexes ofrefraction.
 11. The device of claim 10, wherein a width of the secondwaveguide changes along a length of the second coupling portion.
 12. Thedevice of claim 11, wherein the width_gradually changes by wideningalong a direction of propagation of the first photon and the secondphoton within the second waveguide.
 13. The device of claim 10, whereinthe first photon and the second photon are coupled into the secondwaveguide at different locations in the second coupling portion.
 14. Thedevice of claim 10, wherein the first photon is coupled into the secondcoupling portion before the second photon.
 15. The device of claim 10,wherein the first waveguide layer is made of periodically poledpotassium titanyl phosphate.
 16. The device of claim 10, wherein thesecond waveguide layer is made of silicon nitride.
 17. A methodcomprising: generating a first photon and a second photon in a firstwaveguide formed in a first waveguide layer, where the first photon isin a first mode and the second photon is in a second mode orthogonal tothe first mode; coupling the first photon from the first waveguide intoa second waveguide at a first location within a coupling portion of thesecond waveguide, wherein the coupling portion is a section of thesecond waveguide proximate to the first waveguide; and coupling thesecond photon from the first waveguide into the second waveguide at asecond location distinct from the first location within the couplingportion, wherein the first photon and the second photon are coupled intoone of the first location and the second location based on whetherpropagation within the first waveguide is in the first mode or thesecond mode.
 18. The method of claim 17, wherein a width of the secondwaveguide changes along thea length of the coupling portion by wideningalong a direction of propagation of the first photon and the secondphoton within the second waveguide.
 19. The method of claim 17, whereinthe first waveguide is formed in the first waveguide layer and thesecond waveguide is formed in a second waveguide layer, wherein thefirst waveguide layer and the second waveguide layer are made frommaterials having different indexes of refraction.